Capt. Bill Michaelis said the cause of the fire and how the people were killed are still under investigation.

The Federal Bureau of Alcohol, Tobacco, Firearms and Explosives has joined the investigation.

Troy Donges, who lives across the street, told The Billings Gazette his son was listening to music when he heard a loud bang. He looked out the window, saw flames and woke up his father.

Donges said he banged on the side of the trailer to test the heat and to wake up anybody who might be inside.

"I was hitting the trailer as hard as I could with my hand," Donges said. "There was so much smoke in there, I don't know how anybody could've survived. People told me every time I hit it, it sounded like a gunshot and it woke them up."

Dale Holthues, who lives next door, said his wife called 911 while he ran to the mobile home. He tried to kick in two different doors but couldn't get them open and received no response from inside.

"We could not get any results," he said. "I don't know if they were sleeping or what."